On 14 February, the Altmark entered Norwegian territorial waters at Trondheim. Although Norwegian naval vessels twice stopped the Altmark, Dau hid his ship's guns below and claimed he had no prisoners on board. He resisted any effort to search his vessel on the grounds that she was a German naval ship, immune to search. Despite misgivings and suspecting the nature of the cargo, the Norwegians allowed the Altmark to proceed. Norwegian officials did not want to create an incident that might be used to precipitate a German invasion of their neutral country. Word of events, however, reached the British Embassy at Oslo, and the naval attaché there informed the British Admiralty of the situation. On 16 February 1940, after British planes had located the Altmark, Captain Philip Vian's destroyer flotilla cornered the Altmark near Jössing fjord within Norwegian territorial waters. The Norwegian gunboat Skarv hampered the British Navy's efforts to force Altmark to sea, and the German supply ship then slipped into the fjord.
In London, meanwhile, the War Cabinet met concerning the situation and the reports that the Altmark had on board some 300 British seamen, who were in fact being held below deck in difficult conditions. First Lord of the Admiralty Winston L. S. Churchill personally authorized the boarding and search of the Altmark and liberation of her prisoners.
At 11:00 p.m. on 16 February, Vian's flagship, the destroyer Cossack, entered the fjord. Altmark tried to ram the destroyer, but expert British ship handling saved Cossack from damage. As the two ships brushed together, some of the boarding party leaped across to the German ship. Cossack then again closed, the remainder of the boarding party followed, and Cossack backed clear. In a brief fight, 7 Altmark crew members were killed and 299 British prisoners were freed.
The Altmark incident was definitely an infringement of Norway's neutrality by Britain. Neutral countries could no longer be certain of their inviolability in this war. This incident caused Hitler on 19 February to order an acceleration in his plans to invade Norway, Operation weserübung. After they had conquered Norway, the Germans erected a commemorative marker at Jössing fjord reading (in German), "Here on 16 February 1940 the Altmark was attacked by British sea-pirates."
Roskill, Stephen W. White Ensign: The British Navy at War, 1939–1945. Annapolis, MD: Naval Institute Press, 1960.; Salmon, Patrick, ed. Britain and Norway in the Second World War. London: Her Majesty's Stationery Office, 1995.; Wiggan, Richard. Hunt the Altmark. London: R. Hale, 1982.